Fix properties being lost when reloading placeholder GDScript instance

During reloading in `GDScriptLanguage::reload_all_scripts` a placeholder instance that must remain so is replaced with a new placeholder instance. The state is then restored by calling `ScriptInstance::set` for each property. This does not work if the script is missing the properties due to build/parse failing.
The fix for such cases is to call `placeholder_set_fallback` instead of `set` on the script instance.

I took this chance to move the `build_failed` flag from `PlaceHolderScriptInstance` to `Script`. That improves the code a lot. I also renamed it to `placeholder_fallback_enabled` which is a much better name (`build_failed` could lead to misunderstandings).

modules/gdscript/gdscript.h

@@ -97,6 +97,7 @@ class GDScript : public Script {
 	Ref<GDScript> base_cache;
 	Set<ObjectID> inheriters_cache;
 	bool source_changed_cache;
+	bool placeholder_fallback_enabled;
 	void _update_exports_values(Map<StringName, Variant> &values, List<PropertyInfo> &propnames);
 
 #endif
@@ -209,6 +210,10 @@ public:
 	virtual void get_constants(Map<StringName, Variant> *p_constants);
 	virtual void get_members(Set<StringName> *p_members);
 
+#ifdef TOOLS_ENABLED
+	virtual bool is_placeholder_fallback_enabled() const { return placeholder_fallback_enabled; }
+#endif
+
 	GDScript();
 	~GDScript();
 };